The Physics Analysis of a Gas Attenuator with Argon as a Working Gas

Description

A gas attenuator is an important element of the LCLS facility. The attenuator has to operate in a broad range of x-ray energies, provide attenuation coefficient between 1 and 10{sup 4} with the accuracy of 1% and, at the same time, be reliable and allow for many months of un-interrupted operation. A detailed design study of the attenuator based on the use of nitrogen as a working gas has been recently carried out by S. Shen et al [1]. In this note we assess the features of the attenuator based on the use of argon. We concentrate on the physics ...
continued below

Publisher

Provided By

UNT Libraries Government Documents Department

Serving as both a federal and a state depository library, the UNT Libraries Government Documents Department maintains millions of items in a variety of formats. The department is a member of the FDLP Content Partnerships Program and an Affiliated Archive of the National Archives.

Contact Us

What

Descriptive information to help identify this report.
Follow the links below to find similar items on the Digital Library.

Description

A gas attenuator is an important element of the LCLS facility. The attenuator has to operate in a broad range of x-ray energies, provide attenuation coefficient between 1 and 10{sup 4} with the accuracy of 1% and, at the same time, be reliable and allow for many months of un-interrupted operation. A detailed design study of the attenuator based on the use of nitrogen as a working gas has been recently carried out by S. Shen et al [1]. In this note we assess the features of the attenuator based on the use of argon. We concentrate on the physics issues; the design features will probably be not that different from the aforementioned nitrogen attenuator. Although specific results obtained in our note pertain to argon, the general framework (and many equations obtained) are applicable also to the nitrogen attenuator. In the past, an analysis of the attenuator based on the use of a noble gas has already been carried out [2]. This analysis was performed for an extremely stringent set of specifications. In particular, a very large diameter for the unobstructed x-ray beam was set (1 cm) to accommodate the spontaneous radiation; the attenuator was supposed to cover the whole range of energies of the coherent radiation, from 800 eV to 8000 eV; the maximum attenuation was set at the level of 10{sup 4}; the use of solid attenuators was not allowed, as well as the use of rotating shutters. The need to reach a sufficient absorption at the high-energy end of the spectrum predetermined the choice of Xe as the working gas (in order to have a reasonable absorption at a not-too-high pressure). A sophisticated differential pumping system that included a Penning-type ion pump was suggested in order to minimize the gas leak into the undulator/accelerator part of the facility. A high cost of xenon meant also that an efficient (and expensive) gas-recovery system would have to be installed. The main parameter that determined the high cost and the complexity of the system was a large radius of the orifice. The present viewpoint allows for much smaller size of the orifice, a=1.5 mm. The use of solid attenuators is also allowed for a higher-energy end of the spectrum. It is, therefore, worthwhile to reconsider various parameters of the gas attenuator for these much less stringent conditions. As a working gas we consider now the argon, which, on the one hand, provides reasonable absorption lengths and, on the other hand, is inexpensive enough to be exhausted into the atmosphere (no need for recovery). We concentrate on the processes in the main attenuation cell and just outside it, not touching upon a performance of the differential pumping system.

Collections

Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

Office of Scientific and Technical Information (OSTI) is the Department of Energy (DOE) office that collects, preserves, and disseminates DOE-sponsored research and development (R&D) results that are the outcomes of R&D projects or other funded activities at DOE labs and facilities nationwide and grantees at universities and other institutions.